1. Field of the Invention
The present disclosure relates to selecting bandwidth, and, particularly, to bandwidth selection of FM (frequency modulated) signals.
2. Description of the Related Art
FM bandwidth selection can be considered a compromise between over-demodulation distortion and interference caused by an adjacent channel and/or noise. Over-demodulation distortion is caused by a too restrictive band limitation of a desired channel. More particularly, a narrow bandwidth of the intermediate frequency (IF) filter in a receiver can cause distortion. The opposite situation appears in the case of adjacent channel distortion, low signal-to-noise ratio of a desired channel, or both. More particularly, a wider bandwidth of the IF filter may result in adjacent channel or noise leakage.
Adjacent channel power changes in time and makes IF filtration more difficult. Additionally, deviations from the desired FM channel also change in time. Thus, a fixed IF filtration bandwidth often cannot adaptively react in practice and will quickly succumb to distortion. To overcome this problem, high-performance receivers are equipped with chip-sets allowing an adaptive IF filtration strategy. In adaptive filtering, the IF filtering is accomplished through a combination of a fixed filter and a filter selected from a filter bank. The selection is accomplished through an external microcontroller that monitors the incoming signals and chooses one of the filters based on signal quality. In this way, changes in adjacent channel power or deviations from the desired channel can be tracked and corrected.
While adaptive filtering has numerous advantages, there are still problems not resolved in the prior art. Some prior art solutions tend to chose a narrow band and diverge in special field conditions. Other solutions are based on a state machine that evaluates field conditions according to a tentatively defined schema. However, if there is a situation not defined in the schema, the state machine cannot properly react. Still other solutions do not distinguish between adjacent channel interference and over-deviated signals in strong adjacent channel conditions.
Therefore, it would be desirable to have an FM receiver that can automatically adjust the IF (intermediate frequency) bandwidth according to signal reception conditions without having the drawbacks of the prior art.